Aqueous based well treatment fluids are commonly used in drilling, stimulation, completion and workover operations of subterranean formations. Treatment designs typically mandate such fluids to exhibit a certain level of viscosity. Polyacrylamides are often used in such fluids to provide the requisite viscosity. For instance, polyacrylamides are often used to form viscous gels to prevent fluid loss into the formation. In drilling fluids, such polymers serve to suspend solids and assist in floating debris out of the wellbore.
A common stimulation operation is hydraulic fracturing wherein fractures are created which extend from the wellbore into the formation. In this method, a viscous fracturing fluid containing proppant is introduced into the produced fracture under high pressure. The proppant remains in the produced fracture to prevent the complete closure of the fracture and to form a conductive channel extending from the wellbore into the treated formation and to provide a highly conductive pathway for hydrocarbons and/or other formation fluids to flow into the wellbore. In addition to increasing the capability of proppant transport into the fracture during the fracturing operation, viscosifying polymers also reduce friction, control fluid loss and control fracture geometry.
Filtrate from the fracturing fluid ultimately “leaks off” into the surrounding formation leaving a filter cake comprised of fluid additives. Such additives, including the viscosifying polymers used to provide fluid viscosity, are typically too large to penetrate the permeable matrix of the formation. Recovery of the fracturing fluid is therefore an important aspect to the success of the fracturing treatment.
Recovery of the fracturing fluid is normally accomplished by reducing the viscosity of the fracturing fluid (breaking) such that the fracturing fluid flows naturally from the formation. Thus, in addition to facilitating settling of the proppant in the fracture, the breaker also facilitates fluid flowback to the well.
Breakers work by reducing the molecular weight of the viscosifying agent. Common breakers for use in fracturing fluids include chemical oxidizers, such as hydrogen peroxide and persulfates. Chemical oxidizers produce a radical which then degrades the viscosifying agent. This reaction is limited by the fact that oxidizers work in a stoichiometric fashion. In addition, at low temperatures, such as below 120° F., chemical oxidizers are generally too slow to be effective and other catalysts are needed to speed the rate of reaction. At higher temperatures, chemical oxidizers react rapidly and, when they are encapsulated, are released prematurely, potentially leading to catastrophic loss of proppant transport. Since the use of chemical breakers in fracturing fluids at elevated temperatures typically compromise proppant transport and desired fracture conductivity, alternative sources for breakers have been sought.
Recently, interest has focused on slickwater fracturing in the stimulation of low permeability or tight gas reservoirs. In slickwater fracturing, a well is stimulated by pumping water at high rates into the wellbore, thereby creating a fracture in the productive formation. Slickwater fluids are basically fresh water or brine having sufficient friction reducing agent(s) to minimize tubular friction pressures. Slickwater fracturing fluids usually have viscosities only slightly higher than unadulterated fresh water or brine. The characteristic low viscosity of such fluids facilitates reduced fracture height growth in the reservoir during stimulation.
When aqueous fluids (like slickwater fracturing fluids) not containing a viscosifying polymer are used in stimulation, the pressure during the pumping stage is normally lower than that required in fracturing treatments using viscosifying polymers. The frictional drag of the frac fluid is lowered by the presence of the friction reduction agent(s) in the slickwater fluid. Polyacrylamides are often used as friction reducing agents in slickwater fracturing.
While slickwater fluids introduce less damage into the formation in light of the absence of viscosifying polymers, the friction reduction agent, if left in the formation, can cause formation damage. Effective means of degrading friction reduction agents in slickwater fracturing fluids is desired in order to minimize damage to the treated formation.
Lately, “hybrid” fracturing techniques have evolved wherein a conventional gelled and/or crosslinked fracturing fluid is used as a pad fluid which precedes the introduction of a proppant laden slickwater slurry. The relatively high viscosity gelled fluid provides increased fracture width and improved fluid efficiency, thereby mitigating the limitations of slickwater. Unfortunately, however, viscosifying polymers (such as polyacrylamides) used in such viscosified fluids form filter cakes on fracture faces which cause conductivity damage. Since the concentration of proppant in fracturing fluids free of viscosifying polymer is low and results in propped fracture widths typically no greater than one layer of proppant (±0.5 mm), any effective fracture width lost to the deposition of a filter cake often has catastrophic consequences on fracture conductivity.
Polyacrylamides, when used in oilfield applications have been proven difficult to break or degrade with conventional chemical oxidizers. Alternatives have therefore been sought for degrading polyacrylamides used in oilfield applications.